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| -rw-r--r-- | doc/guix-cookbook.texi | 47 |
1 files changed, 40 insertions, 7 deletions
diff --git a/doc/guix-cookbook.texi b/doc/guix-cookbook.texi index b3c3bac971..2e58c6c795 100644 --- a/doc/guix-cookbook.texi +++ b/doc/guix-cookbook.texi @@ -234,10 +234,11 @@ A list structure can be created with the @code{list} procedure: @end lisp @item -The @dfn{quote} disables evaluation of a parenthesized expression: the -first term is not called over the other terms (@pxref{Expression Syntax, -quote,, guile, GNU Guile Reference Manual}). Thus it effectively -returns a list of terms. +@cindex S-expression +The @dfn{quote} disables evaluation of a parenthesized expression, also +called an S-expression or ``s-exp'': the first term is not called over +the other terms (@pxref{Expression Syntax, quote,, guile, GNU Guile +Reference Manual}). Thus it effectively returns a list of terms. @lisp '(display (string-append "Hello " "Guix" "\n")) @@ -248,9 +249,10 @@ returns a list of terms. @end lisp @item -The @dfn{quasiquote} disables evaluation of a parenthesized expression -until @dfn{unquote} (a comma) re-enables it. Thus it provides us with -fine-grained control over what is evaluated and what is not. +The @code{quasiquote} (@code{`}, a backquote) disables evaluation of a +parenthesized expression until @code{unquote} (@code{,}, a comma) +re-enables it. Thus it provides us with fine-grained control over what +is evaluated and what is not. @lisp `(2 a 5 7 (2 ,a 5 ,(+ a 4))) @@ -261,6 +263,37 @@ Note that the above result is a list of mixed elements: numbers, symbols (here @code{a}) and the last element is a list itself. @item +@cindex G-expressions, syntax +@cindex gexps, syntax +@findex #~ +@findex #$ +@findex gexp +@findex ungexp +Guix defines a variant of S-expressions on steroids called +@dfn{G-expressions} or ``gexps'', which come with a variant of +@code{quasiquote} and @code{unquote}: @code{#~} (or @code{gexp}) and +@code{#$} (or @code{ungexp}). They let you @emph{stage code for later +execution}. + +For example, you'll encounter gexps in some package definitions where +they provide code to be executed during the package build process. They +look like this: + +@lisp +;; Below is a G-expression representing staged code. +#~(begin + ;; Invoke 'ls' from the package defined by the 'coreutils' + ;; variable. + (system* #$(file-append coreutils "/bin/ls") "-l") + + ;; Create this package's output directory. + (mkdir #$output)) +@end lisp + +@xref{G-Expressions,,, guix, GNU Guix Reference Manual}, for more on +gexps. + +@item Multiple variables can be named locally with @code{let} (@pxref{Local Bindings,,, guile, GNU Guile Reference Manual}): |